Water-based ink for inkjet recording is used generally in the form of a solution obtained by dissolving a water-soluble dye in either water or an aqueous solution including a water-soluble organic solvent, or a dispersion liquid obtained by dispersing a water-insoluble color material into water or an aqueous solution including a water-soluble organic solvent. On the other hand, ink for inkjet recording, which includes a sublimating dye, has been used for a sublimation transfer dyeing. In the sublimation transfer method, the ink including a sublimating dye is inkjet-printed on a sheet recording medium such as transfer paper, and then a printed surface of the recording medium is superposed on an object to be dyed such as a hydrophobic fiber product, and sublimate-transferred by heating. Properties required for ink for the inkjet recording are as follows:
(1) prevention of nozzle clogging, ejection stability;
(2) high-quality recording images resulting from uniformity in the direction of ink ejected, the ejection amount, and the dot shape;
(3) ink storage stability;
(4) high print density, high-quality image; and
(5) quick drying and fixation of ink.
In an inkjet recording system, ink should be ejected as ink droplets from thin nozzles. Therefore, (1) is particularly important. In particular, when the inkjet recording system is of the on-demand type, the ejection of ink from the nozzles is stopped temporarily even during a continuous operation. This may lead to an ejection failure during a long continuous operation. Unlike a case of using a water-soluble dye, since the ink for inkjet recording for sublimation transfer is applied as a dispersion liquid using a sublimating dye, it tends to cause clogging, e.g., because precipitates (aggregates) are generated by the degradation of dispersibility and moisture is evaporated from the nozzles, resulting in increased viscosity, for example. In particular, when the apparatus is not operated for a long time, clogging is likely to occur due to evaporation of moisture. Therefore, a humectant is added to suppress the evaporation of moisture. Examples of the humectant include a high-boiling water-soluble organic solvent, a solid water-soluble compound and the like. Examples of the high-boiling water-soluble organic solvent include polyhydric alcohol-based compounds such as glycerin and polyethylene glycol; and pyrrolidone-based compounds such as 2-pyrrolidone, and N-methyl-2-pyrrolidone. For the solid water-soluble compound, saccharides, urea-based compounds and the like may be used.
The sublimating dye ink is used generally in the form of a liquid obtained by dispersing the sublimating dye into an aqueous solution including water, a water-soluble organic solvent and a dispersant. However, when the sublimating dye ink is inkjet printed on a sheet recording medium like transfer paper and then a printed surface of the recording medium is superposed on an object to be transferred and the object is dyed by sublimation transfer by heating, or when the sublimating dye ink is inkjet printed on a fabric formed of a hydrophobic fiber material and then the fabric is dyed by heating, the water-soluble organic solvent is evaporated with water during the heating treatment. This point differs from inkjet printing that is used only for general printing purposes. The sublimation transfer dyeing method requires heating for a predetermined time at temperatures higher than the sublimation point of the sublimating dye. Therefore, even a high-boiling organic solvent may be evaporated. Particularly with a high-boiling organic solvent such as glycerin, the temperature decreases to the condensation point shortly after the evaporated solvent is released into the atmosphere, thus causing smoky steam. This is considered to be undesirable because the appearance is degraded significantly. One possibility for avoiding such a problem is to use a humectant that does not cause any smoky steam (see Patent documents 1-2 below, for example).
Patent document 1 recites an ink for inkjet recording for sublimation transfer that reduces environmental pollution and that has favorable redispersibility. The ink contains a sugar alcohol having 4 or more OH groups as a humectant, a dispersant, and a polyoxyethylene alkyl ether-based compound having an alkyl group having a carbon number of 25 to 150. Since ink for inkjet recording for the sublimation transfer in Patent document 1 includes both an anionic dispersant and a compound based on polyoxyethylene alkyl ether having an alkyl group having a carbon number of 25 to 150 for the purpose of improving the redispersibility, the surface tension tends to be decreased. Thus, in a case where a sheet recording medium such as transfer paper or a fabric formed of a hydrophobic fiber material is subjected to an inkjet printing, excessive penetration may be exhibited with respect to the recording medium, and thereby the density on the surface of the recording medium tends to be lower.
Patent document 2 recites a water-based ink for inkjet recording for sublimation transfer, which contains two types of dispersants, polyglycerin having an average polymerization degree of 2 to 12, and polyol. For the two types of dispersants, an anionic dispersant such as β-naphthalene sulfonate formalin condensate, a nonionic dispersant such as phytosterol ethylene oxide adduct etc. are recited. Patent document 3 recites a water-based ink composition. In the composition, the average dispersed particle diameter of a water-insoluble coloring agent is 0.1 to 0.3 μm, the acid value of the styrene-acrylic acid-based copolymer in the form of free acid is 150 mgKOH/g or more and less than 250 mgKOH/g, and the ratio of the styrene-acrylic acid-based copolymer with respect to the water-insoluble coloring agent is 5 mass % or more and less than 20 mass %. Patent document 3 recites that an extremely high recording density can be achieved by setting the average dispersed particle diameter of the water-insoluble coloring agent to a certain and relatively-large range of 0.1 to 0.3 μm. However, since the ratio of the styrene-acrylic acid-based copolymer with respect to the water-insoluble coloring agent is 5 mass % or more and less than 20 mass %, the dispersion stability (ink storage stability) of the water-insoluble coloring agent is not sufficient. Patent document 4 recites an ink containing a particular pigment, namely an ink composition including an acetylene glycol compound as a penetrant and including a styrene-(meth)acrylic acid-based water-soluble resin as a water-soluble dispersant. It is recited that the ink composition including a particular pigment together with a particular ingredient provides an image having excellent friction resistance and color reproducibility, and it obtains advantages as an ultra-penetrating ink composition. The weight average molecular weight of the styrene-(meth)acrylic acid-based water-soluble resin recited in each of Examples in Patent document 4 is limited to 7,000, the acid value is recited as 70 to 150, and the acetylene glycol compound is not specified. In Examples, as the acetylene glycol compound, only “Surfynol 465” is recited.
Patent document 5 recites an inkjet ink containing a disperse dye. The inkjet ink has either a carboxyl group or a sulfonic acid group as an acidic group, and contains a polymer resin in the range of 2 mass % to 10 mass % inclusive by the solid content, the polymer resin has an acid value of not less than 80 mgKOH and not more than 300 mgKOH. The polymer resin is used not as a dispersant but as a binder resin. With the inkjet ink, a high-quality and highly durable image can be formed by only heating a fabric. Patent document 6 recites an inkjet ink for sublimation transfer to a polyester cloth. The ink contains a sublimating dye, an acetylene glycol-based surfactant, a specific water-soluble organic solvent and water. However, the acetylene glycol-based surfactant is not specified, while there is a recitation of only “Surfynol 465” as an acetylene glycol compound in the Examples. For the dispersant of the sublimating dye, an anionic dispersant (sodium salt of β-naphthalenesulfonate formalin condensate) is used in the Examples.